I want to tell you about a specific moment. The kind that’s easy to overlook at the time because it’s not dramatic.
I was playing a board game with my family. One of those games that involves a bit of quick thinking, pattern recognition, seeing several moves ahead. I’d always been fairly good at them. I enjoyed that sharpness.
Sometime in my mid-30s, a few years into the sleep-disrupted years of having small children, I noticed I was slower. Not dramatically, but slower enough to notice. The quickness wasn’t quite there. I’d be working something out and by the time I’d got to the conclusion, someone else had moved on.
I put it down to motherhood. The mental load, the interrupted sleep, the sheer cognitive demand of keeping multiple small people alive and developing. That seemed like explanation enough.
It was only later, after I started going deeper into the research on sleep architecture, that I understood what was actually happening. And it wasn’t just tiredness. It was something more specific than that.
What five and a half years of broken sleep actually costs
I breastfed my two daughters for a combined total of five and a half years. I tandem fed for six months when my younger daughter was born. I was up in the night across those years in the way you are when you have small children who need you — feeding, settling, being present.
By the time the night feeding was finished and everyone was sleeping through, something had changed in how I slept. I was lighter and more easily woken. Full nights didn’t feel as restorative as they should have. I’d wake up having technically slept and feel... okay. Functional, but not restored. The tank never quite full.
And alongside that: the word-finding difficulties that kept ambushing me mid-sentence. The slower thinking. The research that felt harder to hold together than it once had. The board games.
I was also dealing with other things during this period — long Covid, which arrived when my younger daughter was about two and lasted about six months, compounding an already depleted system. And mould in our house, which we later dealt with and which substantially improved the chronic fatigue that had developed. Multiple things compounding each other, none of them fully understood at the time.
What I’ve since learned is that the cognitive friction — the mum brain, as it gets dismissively called — was not just about being tired. It was a specific consequence of years of disrupted sleep in a specific part of the night.
The stage nobody talks about
Sleep is not one thing. It’s a cycle of distinct stages, each doing something different, and the balance between them matters enormously.
There are four stages. The first two are lighter sleep — the transitional stages, the consolidation of memories, the settling of the nervous system. The third stage is what’s called slow wave sleep — deep sleep, delta sleep, whatever you want to call it. And the fourth is REM: the dreaming stage, emotional processing, a different kind of memory consolidation.
Slow wave sleep is the stage that physically restores you. This is where human growth hormone, primarily a repair and regeneration hormone in adults, is released almost entirely. Where the immune system does its most active work. Where cellular repair happens. Where the brain clears metabolic byproducts, including proteins associated with cognitive decline, through a system that is essentially only fully active during this stage.
And here is the crucial detail: slow wave sleep is concentrated in the first half of the night. The early cycles (the first two or three) contain the majority of it. By the second half of the night, slow wave sleep is largely done, and REM sleep dominates.
Which means that if your sleep is interrupted in the first half of the night, by a baby who needs feeding, by pain, by anything, you’re not losing sleep in general. You’re specifically losing slow wave sleep. The stage that rebuilds you. Repeatedly. Over months, years.
And the nervous system adapts. After enough nights of being woken in early sleep, it learns to stay closer to the surface. More vigilant. More ready to respond. This is exactly what happened to me. I became a lighter sleeper, not because anything was wrong, but because my nervous system was doing exactly what nervous systems do when they learn that early sleep isn’t safe to go deep in.
The problem is that this adaptation persisted long after the feeding stopped. Even when there was no longer any reason to be vigilant, my nervous system had learned to stay light. The deep sleep that should have been fully recovering in those full nights just... wasn’t quite doing its job. And that’s what the unrested mornings were. Not insufficient hours, but insufficient depth.
The cognitive piece follows directly from this. Slow wave sleep is when the brain does its deepest clearing and restoration work. Years of reduced slow wave sleep produces exactly that signature: not dramatic cognitive impairment, but a persistent low-grade friction. The word-finding. The slower pattern recognition. Everything slightly harder than it should be. Not mum brain as a vague consequence of being busy. Mum brain as a specific consequence of years of fragmented slow wave sleep.
Why perimenopause hits sleep first — and why nobody tells you
I’m going to share something here that I think most women in their 40s have never been told, and it changes how the whole picture makes sense.
Most of the conversation about perimenopause and sleep focuses on hot flushes and oestrogen. And that’s real: oestrogen decline does significantly affect sleep. But there’s an earlier chapter that almost never gets explained.
Progesterone declines before oestrogen. In perimenopause, progesterone is typically the first hormone to start dropping, often years before oestrogen begins its more dramatic decline. And progesterone has a direct, specific relationship with slow wave sleep.
Progesterone and one of its metabolites, allopregnanolone, act on GABA receptors in the brain. These are the same receptors that sleep medications target. They promote deeper sleep. They’re essentially a natural slow wave sleep protector. When progesterone starts to decline, that protection reduces. Slow wave sleep becomes more fragile. Women start sleeping more lightly, waking more easily, not feeling rested despite adequate hours.
This can happen years before the hot flushes start. Years before a woman would think to say “I think this might be perimenopause”. She’s not having night sweats. Her periods might still be regular. She just... isn’t sleeping as well. And she doesn’t know why.
Sleep is often the first perimenopause symptom. Almost nobody tells women this.
The oestrogen picture comes later, affecting the overall stability of sleep architecture, temperature regulation, and the cortisol pattern. The 3am waking becomes more common. The second half of the night gets harder. But the earlier, quieter deterioration, the slow wave sleep loss from progesterone decline, that’s what sets the stage.
What I haven’t told you
Everything I’ve described (the slow wave sleep restoration, the cognitive clarity gradually returning, the mornings starting to feel different) happened because I gave my body clock the conditions it needed. Consistently, over months.
But here’s where I see women get stuck. They improve one thing — morning light, or sleep timing, or nutrition — and they feel better for a while, then plateau. Usually because the other pieces are still misaligned. Slow wave sleep restoration doesn’t happen in isolation. It happens when the whole system is working together: the circadian signal setting the cortisol curve correctly, the evening darkness allowing melatonin to rise, the meal timing aligning the gut clock, the nervous system feeling safe enough to go deep.
If you want to understand how the four pillars work together, the free masterclass at francesnorgate.com/masterclass is the place to start — about an hour, free, brings everything into one picture.
Where to start
Morning light: this is still the foundation. Going outside within an hour of waking, for at least twenty minutes, is the single most powerful thing you can do for sleep quality. Not because of some vague wellbeing benefit, but because it sets the cortisol curve correctly, which sets the conditions for deep sleep in the first half of the night twelve hours later.
Evening darkness after 8pm: dimming lights and reducing screen brightness allows melatonin to begin rising at the right time. The depth of your slow wave sleep is partly determined by how cleanly the evening-to-night transition happens.
And if you’ve had years of disrupted sleep, whether from feeding babies, chronic stress, illness, or anything else that kept your nervous system vigilant, be patient with the restoration. The nervous system learned to be light over a long period. It unlearns on a similar timescale. Months, not weeks. The changes are real. They just take longer than you’d hope.
The sleep that restores you is available. Your body knows how to produce it. It just needs the conditions that make it safe to go there.
Thank you for reading,
Frances
